Lasers
Gas, solid-state, semiconductor and dye laser technologies
Introduction
The word laser is an acronym for Light Amplification by Stimulated Emission of Radiation. A laser is a source of electromagnetic radiation with properties that may include a narrow wavelength range, high directionality, polarisation and coherence. The exact properties depend on the particular laser system.
Essential Parts of a Laser
A laser requires an active medium with suitable quantum-energy levels. The medium may be a gas, single crystal, liquid dye, semiconductor or, in some specialised systems, a beam of free electrons. Energy must be supplied to create a population inversion. Pumping may be electrical, optical, chemical or another form appropriate to the medium.
Most conventional lasers also use an optical resonator formed by two mirrors. One mirror is highly reflective, while the output coupler transmits a small part of the circulating light. The resonator amplifies selected optical modes and defines the output beam. Semiconductor lasers normally require collimating optics because the raw diode output is strongly divergent.
Laser Types and Active Media
There are many laser families: atomic, ionic and molecular gas lasers; solid-state lasers; dye lasers; semiconductor lasers; excimer lasers and others. Examples of active media include CO₂, N₂, He–Ne, He–Cd, argon, hydrogen, carbon monoxide, ArF, KrF, XeCl, ruby, rhodamine 6G, fluorescein, coumarin dyes, copper vapour, iodine, gold vapour, alexandrite, Ti:sapphire, rare-earth-doped crystals such as Nd:YVO₄ and semiconductor compounds such as GaAs, AlGaAs, AlGaInP and GaN.
Cooling requirements depend heavily on output power and efficiency. Small diode lasers may need only a modest heat sink, while high-power gas and solid-state systems require forced-air or water cooling.
Optical Power and Wavelength
Laser output can range from microwatts to extremely high peak powers in pulsed research systems. Semiconductor laser diodes are available from milliwatts to many watts. They are found in laser pointers, printers, optical drives and DPSS systems.
A green DPSS laser does not normally contain a green laser diode. An 808 nm infrared diode pumps an Nd:YVO₄ or similar crystal, producing 1,064 nm radiation, which is then frequency-doubled in a nonlinear crystal such as KTP to 532 nm. Conversion losses are significant, so the infrared pump power is much higher than the visible green output.
He–Ne lasers are valued for beam quality and stability. Common units emit red light, while less common variants produce green, orange or infrared wavelengths.
Laser Safety
Even a 5 mW visible laser can damage eyesight under unfavourable conditions. Higher-power lasers can cause immediate and irreversible retinal injury, ignite dark materials and create hazardous specular reflections. Invisible infrared and ultraviolet beams are especially dangerous because the eye provides no reliable warning.
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